Differential pressure thermoforming apparatus for forming large articles such as trailer bodies and boats

ABSTRACT

Differential pressure thermoforming apparatus for forming large articles such as trailer bodies and boats in relatively thin sheets of thermoplastic material comprising a sheet supporting box frame for clamping the edge portions of a sheet of thermoplastic material and defining a cavity spanned by the clamped sheet. Load assist bars are disposed in the cavity to initially support the mid-portion of the thermoplastic sheet while it is being clamped on the frame and then fluid under pressure is admitted to the cavity to lift the midportion of the sheet upwardly off the bars. A sheet heating oven is mounted laterally adjacent one side of the mold box frame and is movable into superposed relation with the box frame where it is then tilted so that the sheet carried thereby is heated to a substantially uniform temperature. The clamped sheet, having its mid-portion supported by fluid pressure, is then moved upwardly toward a differential pressure mold to form a shape in the sheet of material. Doors are provided on the sides of the mold box frame and are opened to exhaust cooling air which is introduced to the cavity. After the shape is formed and the mold box frame is moved away from the mold, the load assist bars function to eject the molded part from the frame.

United States Patent 1 91 Rise [ 1 Jan. 30, 1973 [541 DIFFERENTIALPRESSURE THERMOFORMING APPARATUS FOR FORMING LARGE ARTICLES SUCH ASTRAILER BODIES AND BOATS [75] lnventor: Donald J. Rise, Box 321, MountPleasant, Mich. 48858 t [22] Filed: Oct. 22, 1970 [21] Appl. No.: 83,099

[52] U.S.Cl ..425/384, 425/388 [51] Int. Cl ..B29c 17/00 [58] FieldofSearch ..18/19 F, 19F

[56] References Cited UNITED STATES PATENTS 2,952,875 9/1960 Herrick..18/19 F 3,568,253 3/1971 Schwartz et al ..18/19 F 3,307,222 3/1967Baldwin et a1 ..18/19 F 3,600,753 8/1971 Otto ..18/19 F 2,160,182 5/1939Vrooman ..18/19 F 3,007,206 11/1961 Griswold ..18/19 F UX 3,377,6564/1968 Tilden ..18/19 F 3,566,650 3/1971 Johnson ..18/19 F X 3,597,7998/1971 Earle ..18/19 F X 3,341,895 9/1967 Sheby ..18/19 F 3,441,9835/1969 Cheney ..18/19 F 3,543,335 12/1970 Meyer ..18/19 P X 3,540,11311/1970 Krutzikowsky ..18/19 F X 3,267,521 8/1966 Kostur ..18/19 FPrimary Examiner-J. Howard Flint, Jr. Att0rneyLearman & McCulloch [57]ABSTRACT Differential pressure thermoforming apparatus for forming largearticles such as trailer bodies and boats in relatively thin sheets ofthermoplastic material comprising a sheet supporting box frame forclamping the edge portions ofa sheet of thermoplastic material anddefining a cavity spanned by the clamped sheet. Load assist bars aredisposed in the cavity to initially support the mid-portion of thethermoplastic sheet while it is being clamped on the frame and thenfluid under pressure is admitted to the cavity to lift the midportion ofthe sheet upwardly off the bars. A sheet heating oven is mountedlaterally adjacent one side of the mold box frame and is movable intosuperposed relation with the box frame where it is then tilted so thatthe sheet carried thereby is heated to a substantially uniformtemperature. The clamped sheet, having its mid-portion supported byfluid pressure, is then moved upwardly toward a differential pressuremold to form a shape in the sheet of material. Doors are provided on thesides of the mold box frame and are opened to exhaust cooling air whichis introduced to the cavity. After the shape is formed and the mold boxframe is moved away from the mold, the load assist bars function toeject the molded part from the frame.

22 Claims, 8 Drawing Figures PATENTEUJAH 30 I975 SHEET 10F 5 n3 45b noFIG. 2

0) F3 3 f; 9 3 g S INVENTOR DONALD J. RISE lawman MCCulloch ATTORNEYSPAIENTEDJAHBO ms 3. 713; 765 sum 2 or 5 5 INVENTOR DONALD J. RISE ZaTmanMcCullnch ATTORNEYS PATENTEDJAn 30 IIO DONALD J RISE BY 7 v Idiman 6McCullnch PATENIEDJAHSO I973 I 3,713,765 sum u or 5 FIGJT INVENTORDONALD J. RISE Iearman f5 McCuHsclz ATTORNEYS SHEET 5 [IF 5 S- u L IATTORNEYS DIFFERENTIAL PRESSURE TI'IERMOFORMING APPARATUS FOR FORMINGLARGE ARTICLES SUCH AS TRAILER BODIES AND BOATS FIELD OF THE INVENTIONThis invention relates to differential pressure thermoforming machinesfor forming shapes in thin sheets of thermoplastic material by reversedrape molding, and more particularly, to new and improved apparatus forhandling and supporting and heating relatively thin sheets ofthermoplastic material, and new and improved apparatus for cooling theparts formed in the sheets.

I BACKGROUND OF THE INVENTION Thermoforming machines of the typedisclosed herein commonly employ a sheet heating apparatus or bankdisposed above and/or below the level of the plastic sheet of materialin order to bring the plastic to the forming temperature. In somemachines the heating oven is moved from a remote position over to aposition above the sheet or web and, before and after the oven is movedabove or below the sheet of material, it is adjacent one side of thesheet so that one side of the thermoplastic sheet is heated to a highertemperature than the other side. Thus, when the heating array is broughtinto superposed relation with the plastic sheet, all portions of thesheet do not reach the forming temperature at the same time. It is anobject of the present invention, therefore, to provide differentialpressure thermoforming machines having a tiltable heating bank to heatthe sheet of thermoplastic material, in which a shape is to be formed,to a substantially uniform temperature.

Thin sheets of thermoplastic material have relatively little hotstrength." When a thin sheet of thermoplastic material is clamped alongits edges and used for forming relatively large articles, the strengthof the heated thermoplastic material is insufficient to support themid-portion of the sheet. Accordingly, it is an object of the presentinvention to provide new and novel means for supporting the mid-portionof a heated thermoplastic sheet while it is being moved to a formingposition.

Forming machines of this character usually employ at least one diemember to form shapes in the preheated sheets of material, whichnaturally sag when heated. If a male die member is disposed above thesheet of material and the sheet of material is moved upwardly intoengagement therewith, the sagging sheet will engage the perimetralportions of the die member before the middle portion thereof is engagedby the sheet. The sheet thus seals itself around the male die memberperimeter and locks in any air therebetween. When the middle portion issubsequently brought into engagement with the male die member, theexcess material overlaps and forms a double thickness webbing whichdetracts from the appearance and results in an article which must bescrapped.

Accordingly, it is an object of the present invention to providedifferential pressure thermoforming apparatus for forming articleswithout the formation of center webbing caused by excessive material ona plane surface.

It is a further object of the present invention to provide a new andimproved apparatus for cooling a part formed in a sheet of thermoplasticmaterial.

Still another object of the present invention is to provide differentialpressure thermoforming apparatus which includes apparatus that willsupport the mid-portion of the heated sheet in a sheet supporting boxframe prior to the formation of a part therein and also functions toeject a part molded in the sheet.

In accordance with the present invention, there is provide differentialpressure thermoforming apparatus comprising a sheet supporting box framemeans including supporting means for supporting edge portions of a sheetof thermoplastic material and defining a cavity which a sheet ofthermoplastic material spans when supported by the supporting means,differential pressure mold means for forming shapes in sheets ofthermoplastic material, means for relatively moving the sheet supportingmeans and the mold means between remote and operative molding positionsso that a sheet clamped on said clamping means is brought intoengagement with the mold means to form a shape in said sheet, heatingmeans for heating the sheet of thermoplastic material, and means forintroducing fluid under pressure to the cavity in said mold box framemeans to support the midportion of a sheet on the box frame as the sheetis being heated.

The present invention may more readily be described by reference to theaccompanying drawings, in which:

FIG. 1 is a side elevational view of the differential pressurethermoforming apparatus formed according to the invention, parts beingbroken away to more clearly illustrate certain portions thereof;

FIG. 2 is a top plan view of the apparatus illustrated in FIG. 1;

FIG. 3 is a sectional end view taken along the line 3--3 of FIG. 2;

FIG. 4 is a fragmentary end elevational view, particularly illustratingthe sheet supporting carriage, which is taken along the line 44 of FIG.1;

FIG. 5 is a section front view, particularly illustrating the heatingapparatus, which is taken along the line 5- 5 of FIG. 2',

FIG. 6 is a sectional front view taken along the line 6-6 of FIG. 2 andparticularly illustrating the mechanism for cooling the parts formed;

FIG. 7 is a sectional, front elevational view, taken along the line 7-7of FIG. 4, particularly illustrating the sheet clamping structure andthe door opening mechanism; and

FIG. 8 is a schematic diagram of an electrical control circuit forcontrolling the operation of the apparatus illustrated in FIGS. 1-7.

Referring now more particularly to the drawings wherein a preferredembodiment of the invention is illustrated, the apparatus formedaccording to the invention is mounted on a frame, generally designatedF, including upper and lower side rails 10 and 12 connected byvertically extending posts 14 and end members 16.

Two identical forming stations, generally designated 18 and 20, areprovided on the frame F on opposite sides of a central station 22. Thereis a movable heater assembly 24 which is movable from the centralstation 22 alternately to the forming stations 18 and 20.

Spanning the side rails in each of the mold stations 18 and 20 are pairsof eye beams 26 which support a stationary platen 28 mounting a malemold 29. Vacuum ports 30 are provided in the mold 29 and are adapted toselectively communicate with a suitable vacuum source (not shown), forcreating a differential pressure on opposite sides of a sheet ofthermoplastic material brought into engagement with the male mold 29,and an air pressure source (not shown) for ejecting the molded partstherefrom.

Provided at each of the forming stations 18 and 20 is a verticallymovable plastic sheet supporting sealed box frame, generally designated32, including a pair of side walls 36 connected by a pair of end walls34 and a pair of intermediate wall cross members 35. The walls 35 and 36include upper inwardly extending terminal flange portions38 providingsupporting surfaces for a sheet of thermoplastic material P which isclamped on the flange portions 38 by a plurality of solenoid actuated,pneumatic cylinders 40 supported on support members 42 (FIG. 7)extending outwardly of the wall members 35 and 36. Sheet clamping bars44 are connected with one end of at least one of a plurality of bellcranks 46 which are pivoted on the support 42 at 48 and connected attheir opposite ends with the piston rods 40a of the cylinders 40.Typical sheets of thermoplastic P utilized are of the .ABS type and are88 inches wide by 158 inches long and have a thickness typically rangingbetween 0.070-0. 150 inches.

The sheet supporting box frame 32, which is mounted for verticalmovement along guide rods 102, is provided with a bottom wall 50spanning the walls 35 and 36. A plurality of bores 78 are provided atopposite ends of the bottom wall 50 and slidably receive a pair ofsupporting rods 80 which threadedly receive stop members 81 that rest onchannel member 1 la in their lowermost position, as shown in FIG. 3, andmount, at their upper ends, crosswisely extending load assist bars 82 at30-40 inch intervals for supporting a sheet of plastic P clamped on theside frame members 35 and 36. The rods 80 also threadedly receive stopmembers 84 provided with resilient foam cushion pads 86 on the uppersurfaces thereof for engaging the bottom wall 50 and sealing theopenings 78 around the rods 80 when the mold box frame 32 is in thesolid line position shown in FIG. 3. At their lower ends the rods 80also threadedly receive stop members 83 for engaging the channel members11a as the rods 80 are moved upwardly to limit the vertical movement ofthe rods 80.

For raising the plastic sheet 1 off the load assist bars 82 andsupporting it in the chain line position shown in FIG. 3, the bottomwall 50 is also provided with a plurality of apertures 71 communicatingwith an air supply line 71a connected with a suitable source of airwhich is provided under a pressure of approximately 80 pounds per squareinch and traveling at about 20 cubic feet per minute. Deflecting baffleplates 75 are supported over the openings 71 by pins 75a to deflect theair stream passing through the openings 71 so as to not directly impingeon the plastic sheet P so as'to form cold spots therein. When a sheet Pis clamped to the flanges 38 on the walls 35 and 36, a substantially airtight seal is formed so that when air is admitted to the cavity formedby the walls 34, 36 and 50, the plastic sheet P is moved upwardly to thechain line position.

For/admitting cooling air to the cavity in the mold box frame to cool apart formed in the plastic sheet P, a plurality of openings 52 areprovided in the bottom wall 50 and are spanned by frame members 54disposed below the wall 50. Octagonally shaped openings 54a are providedin the frame members 54 and receive octagonally shaped shrouds 56 whichmount fan driving motors 58 having fan blades 59 mounted on the terminal,ends of the motor shafts 58a. Pairs of doors 60, pivotally mounted onpairs of shafts 61 and spanning I the octagonally shaped shrouds 56, areprovided to close the octagonally shaped openings defined by the shrouds56. The doors 60 are pivoted from the solid line positions shown in FIG.6 to the chain line positions, also shown in FIG. 6, by bell cranks 62positioned on opposite sides of the shrouds 56 and pivoted on pivot pins63 provided on the frame member 54. One end of each bell crank 62 isconnected with the portion 60a of each of the doors 60 by a link 66passing through a suitable opening in the bottom wall 50. Links 68connect the opposite ends of the bell crank arms 62 with blocks 69 fixedto a pair of shafts 70 which are journaled in suitable bearing blocks 72attached to the box frame walls 36. Torsion springs 74 (FIG. 4) areplaced on the shafts 70 and one end 74a of each torsion spring issuitably fixed to the wall 36 and the opposite end 74b of each torsionspring is engaged with a link 76 fixed at one end to the shaft 70 andmounting at its other end a cam follower roller 78. As the sheetsupporting box 32 is moved upwardly from the position shown in FIG. 1,in a manner to be later described, the cam follower rollers 78 engagestationary cam members 79 (FIG. 2) provided on the upper side framemembers 14 so as to pivot the shafts 70 (see the arrow a in FIG. 6) tomove the doors 60 from the solid line positions shown in FIG. 6, tothechain line positions also shown in FIG. 6. The fan motors 58 are thenenergized in a manner which will be later described to draw cooling airinto the sealed cavity formed by the walls 34, 35 and 50 in the mold box32.

The side walls 36 are also provided with apertures (FIG. 7) which areselectively closed by hinged doors 92 pivotally connected with the wall36 by pivot pins 93. A plurality of links 95, which are each fixed atone end to the shafts 70, are pivotally connected with links 94 that arepivotally connected with rods 96 extending from the doors 92. When thecarriage 32 is moved upwardly so that the cam follower members 78 engagethe cam surfaces 79, the shafts 70 will pivot to swing the doors 92outwardly so that the cooling air introduced by the fans 59 will beexhausted through the ports 90.

To move the carriage assembly 32 from the positions shown in FIG. 1 toraised positions adjacent the male mold 29, two solenoid actuated,spring returned, fluid operated, cylinders 98 are pivotally mounted tothe frames with pivot pins 99 (FIG. 3) at each station 18 and 20, andinclude piston rods 100 connected with plates 101 that are welded orotherwise suitably secured to the cross frame members 34 and 35 onopposite ends of the carriage 32. I

The guide rods 102, which are fixed to the upper and lower frame members12 and 10, are slidably received in the plate 101 spanning the end rails34 and 35. Another pair of rods 103 may suitably be journaled on theframe member F and mount suitable stop members 103a (FIG. 1) thereon tolimit the upward and downward travel of the member 32 as desired.

Journaled in suitable bearings provided in the side rails 36 at oppositeends of the mold box frame 32 is a pair of shafts 104. Mounted on oneend of each of the shafts 104 is a sprocket 105 having a chain 106trained therearound. Also mounted on opposite ends of the shafts 104 arepairs of spur gears 108 (FIGS. 1 and 2) in riving engagement with racks109 fixed to the frame F in any suitable manner such as by welding.Guide rollers 109a, which are rotatably mounted on shafts provided onthe side walls 36, engage the opposite sides of the racks 109 so as tosandwich the racks 109 between the gears 108 and the guide rollers 109a.As the cylinders 98 are actuated to move the piston rods from solid linepositions shown in FIG. 3 to the chain line positions shown in FIG. 3,the gears 108 engage the racks 109 to drive the sprockets and the chain106 so that all sides of the box frame 32 move upwardly in unison. Whenthe mold member 29 and the sheet clamping frame 32 are spread apart tothe positions shown in FIG. 1, the heating assembly 24 is movable fromthe solid line position shown in FIG. 1 to the chain line position, alsoshown in FIG. 1, between the mold member 29 and the mold box frame 32.

The heating assembly or oven 24 comprises a horizontally movable outermain frame 110, pivotally supporting an inner frame I. The main frame110 includes end walls 111 and side walls 112 and is supported forlongitudinal movement on a pair of tracks 113, which extend the entirelength of the machine, by guide wheels 115 journaled on shafts 116provided on support bars 114 that are welded or otherwise suitablysecured to the frame wall members 111 and 112. Welded to the side framemembers W of the pivotal frame I are pairs of vertically extendingchannel shaped members 117a connected by cross members 117. Spanning thecross members 117a positioned on opposite side walls 112 are upper andlower channel shaped members 119 and 120 respectively.

The pivotal frame I includes side walls W connected by rectangularlyshaped channel members 127 having a plurality of upper reinforcing crossmembers and 126 on opposite ends thereof. A shaft 122, which passesthrough the middle channel member 127 and is journaled in the side wallsW with bearings (not shown), is also journaled at its ends in suitablebearings (not shown) provided in the walls 112 of the main heaterassembly frame 110. Extending within the tubular cross members 126 are aplurality of commercially available electrical resistance heatingelements 130 (FIG. 5) adapted to be connected to a source of alternatingcurrent with suitable connectors, such as a rolling contact traveling onan electrically energized track.

To maintain the relative horizontal position of the heating oven 24,centering springs 132 are fixed to the underside of the channel members120 by bolts 133 connected thereto by nuts 134. The springs 132 bearagainst the upper reinforcing members 125 to maintain the position ofthe inner frame I. Disposed on opposite ends of the main frame 112 aboveeach of the cross members 127 are pairs of cam plates 138 adapted toengage the rollers 139 journaled on the cross members 16 of frame F,when the heating assembly 24 is moved from the central position 22 toeither of the forming stations 18 and 20. When the rollers 139 areengaged by one of the cam plates 138 to force the oven to the chain lineposition shown in FIG. 5, the oven is tilted about the shaft 122 againstthe force of one of the springs 132. At least one shock absorber 140 isconnected between the top plate 125 and the cross member 119 so that asthe tilt ramp 138 is moved off the follower roller 139, the heating unit24 does not hunt and any oscillations are quickly damped out.

To move the oven or heating unit 24 from the central station 22 to theforming stations 18 and 20, a cable is connected at opposite ends 145aand 1451; to the main frame 110 of the heating unit 24 and is trainedaround suitable pulleys 146, 147 and 148 journaled on the frame F andthence through a cylinder 150 wherein a double acting, solenoidactuated, fluid operated piston 152 is mounted and has a piston rod 153connected with the cable 145 by suitable pin connection mechanism (notshown). As the piston 153 is moved in the direction of the arrow b, theoven assembly 24 is moved in the direction of the arrow c. Conversely,when the piston 153 is moved in the direction of the arrow 0, the oven24 is moved in a direction of the arrow b toward the forming station 20.

' THE CONTROL CIRCUIT Referring now to FIG. 8, a schematic diagram of acontrol circuit for operating the apparatus illustrated in FIGS. 1-7 isdepicted. For purposes of clarity, only that portion of the circuitwhich is used to operate the heating assembly 24 in conjunction with theforming station 18, will be illustrated and described. However, itshould be realized that the circuit is duplicated for the formingstation 20.

The lines L-l and L-2 are connected across a suitable source of power,such as l 10 V, 60 cycle, alternating current. The circuit andsub-circuit lines connected between the lines L-l and L-2 are designatedL-3 through L-19. Starting switch members S1, S2, S3 and S4 areconnected in lines L-3, L-8, L-10 and L-ll, respectively. Connected inseries circuit with the switch S1 in line L-3 is a holding relay CR,which is operable to close a set of normally open contacts CR1, (lineL-4) connected in parallel with the switch S1, and the normally closedcontacts LS-la of a limit switch LS-l which is mounted on the frame Fand is actuated by the mold box frame 32 when the frame 32 isapproximately 10 inches from its lowermost position as shown in solidlines in FIG. 3. The limit switch LS1 is of the type which is onlyactuable when the mold box frame 32 is moving downwardly and is notaffected when the mold box frame 32 moves upwardly. Connected inparallel with the holding relay CR in line L-5 is the advance solenoid40 for directing fluid to the clamp advancing cylinders 40 so as to movethe clamp bars 44 to the clamping position as shown in FIG. 7. When asheet of thermoplastic material P is clamped to the frame 32, asubstantially air tight enclosure is formed. The starting switch S2 isconnected in series with a timer clutch CLl for a timer motor M1 whichis connected in line L-6. The timer clutch CLl, when energized, isoperable to close sets of normally open contacts CLla and CLlb connectedin series circuit relation with the timer motor M1 in line L6. Connectedin parallel with the timer clutch CLl is a photoelectric cell BE in lineL-8, including a set of contacts EEl in L-9. The photoelectric cell EEis mounted on one side of the frame 32 and includes a conventional lightsource for transmitting a light beam to a reflector 160 mounted on theopposite side of the frame 32 for reflecting a beam 162 (FIG. 2) to thephotoelectric eye EE. The photoelectric eye EE is mounted such that thebeam 162 will be interrupted when the plastic sheet P has been billowedupwardly approximately 1 94 inches from the solid line position shown inFIG. 3 to the chain line. position also shown in FIG. 3.

The contacts EEl which are closed when the beam 162 is transmittedwithout interruption, is connected in series with a air billowingcontrol solenoid 164, which when energized, is operable to direct air tothe cavity formed in the sheet supporting box 32 through the ports 72against a deflecting plate 76. When the sheet P has billowed up to theposition shown in chain lines in FIG. 3, the electric eye contacts EE1will open to deenergize the solenoid 164.

The electrically operably timer Ml, of the type manufactured by EagleManufacturing Co. of Moline, Illinois, under the trademark CYCLEFLEX,includes sets of timer switch contacts M la, M lb and M10 in lines L-l3,I .12 and L-15, respectively, which close at varying times. The switchcontacts Mla include a movable armature 165 which is initially inengagement with a fixed contact 166 and is operable to move intoengagement with a fixed contact 168 after the motor for the timer M1 hasbeen operating for a predetermined time. The normally open switchcontacts Mlb are connected to the fixed contact 166 and the normallyopen limit switch contacts LS-2a of a limit switch LS-2 (FIG. 1) whichis actuated by the box frame 32 when it is in its lowermost position.The switch contacts Mlb and LS-2a are connected in series circuitrelation with the advance solenoid 152a for actuating an air valve whichwill permit air to flow to the piston 152 so as to move the heatercarriage 24 in the direction of the arrow c to a position between themale mold member 29 and the mold box frame 32. A limit switch LS-3mounted at the forming station 18 (FIG. 2) includes a set of normallyopen contacts LS-3a (line L-14) which are closed when the heatingassembly 24 is moved to the forming station 18. The contacts LS-3a areconnected in series with the return solenoid 15% of the cylinder 152 formoving the heater carriage 24 in the direction of the arrow b to itscenter position. The limit switch LS-3, when actuated, is operable toopen a set of normally closed contacts LS-3b in line L-l5 which areconnected in series circuit relation with the timer contacts M1 and theraise solenoid 98a for actuating an air valve which will permit air toflow to the cylinder 98 so as to raise the clamp box 32 from theposition shown in solid lines in FIG. 1 to the operative moldingposition adjacent the mold 29. Connected to the junction 169 (line L-l)is a timer clutch coil CL2 (line L-ll) for the timer M2 (line L-) andthe start switch S4. When energized, the clutch coil CL2 is operable toclose a set of contacts CL2a in line L-10. The start switch S3 is alsoconnected in series circuit with the contacts CL2a and a secondelectrically operable forming timer motor M2, such as that made by EagleManufacturing Company under the trademark MUL- TIFLEX. The clutch coilCL2 for the timer M2 (line L-l0) is connected to the junction 169 inline L-15 and is also connected in series circuit relation with thestart switch S4 in line L-ll. The clutch coilCL2 includes a set ofnormally open contacts CL2a which are closed when the clutch coil CL2 isenergized. The contacts CL2a are connected in series circuit relationwith the switch S3 and an electrically operable forming timer M2 whichincludes sets of normally open contacts M2a, M2b and M20 in lines L-16,L-l7 and L-18, respectively, which are closed at various intervals asthe timer operates.

The normally open timer contacts M2a are connected in line L-16 with thenormally open limit switch contacts LS-Sa of a limit switch LS-5 whichis located adjacent the male mold 29 and is actuated by the mold boxframe 32 when it is in the operative molding position. The contacts M2aand limit switch contacts LS-5a are connected in series circuit relationwith a solenoid 170 of a Johnson poppet type valve 170a manufactured byJohnson Corporation, of Three Rivers, Michigan, which, .whenactuated, isoperable to connect the ports 30 in the male mold 29. with a source ofvacuum (not shown). After a part has been. formed in the sheet ofplastic P, the timer contacts M2a open and the timer contacts M2c inline L-18 close to energize a solenoid 172 connected in line L-18 whichoperates the valve 170a that disconnects the ducts 30 in the male die 29from the vacuum source and connects them with a source of air to ejectthe part formed away from the male mold 29. Prior to the contacts M2cclosing, the contacts M2b in line L-l7 closed to energize the armatures58a of the fan motors 58 which draw air through the pivotal doors 60 tocool the part thus formed. The heater elements are connected across thesource in line L-l9.

THE OPERATION With the mold box frame 32 in the position shown in FIG.1, a sheet of plastic P is placed on the flange portion 38 of the moldbox frame 32 and the start switch members S1 through S4 are manuallyclosed. When the switch S1 is closed, the clamp solenoid 40 is energizedto direct air to the cylinder 40 in such a manner asto move the clampbars44 to the position shown in FIG. 7 to clamp the edge portions of thesheet P between the bar 44 and the flanges 38.

As soon as the starting switch S2 is closed, the electric eye EE isenergized to close the contacts EEl in line L-9 to energize the solenoid164 so as to direct air through the ports 72 to move the sheet Pupwardly off the cross members 82 to the chain line position shown inFIG. 3. When the sheet P is billowed upwardly, the light beam 162 isinterrupted to cause the contacts EEl to open. At This time, the sheet Pis wrinkled, however, when the heating assembly 24 is moved intosuperposed relation with the carriage 32 to heat the sheet P, theincreased temperature of the billowing air will expand and pressurizethe entrapped air which forces the sheet P outwardly until it is smooth.

With the closing of the switch S2, the clutch coil CLl is energized toclose the contacts CLla and CLlb in line L-6 to start the heating timerM1. With the mold box frame 32 in the lowermost position to actuate thelimit switch LS-2, the limit switch contacts LS-2a in line L-12 areclosed to energize the solenoid 152a when the timer contacts Mlb of thetimer are closed. When the solenoid 152a is energized, the piston 152 isoperable to move the heating assembly 24 from the solid line positionshown in FIG. 1 to the chain line position, also shown in FIG. 1. Whenthe carriage 24 is moved to the far left position as shown in FIG. 5,the inclined ramps 138 will engage the rollers 139 on the frame F,causing the heating oven 24 to tilt about the shaft 122 to the chainline position shown in FIG. 5. When the sheet P has been heated for asufficient time, Le, 25 seconds, the timer contacts Mlb open and theswitch arm 165 of the timer contacts Mlamoves into engagement with thefixed contact 168to energize the solenoid 152b in line L-14 through thecontacts LS-3a which are closed when the heating assembly moves to theforming station 18. When the solenoid 152b is energized, the heatingassembly 24 returns to the solid line position shown in FIG. 1 where itis halted when the limit switch contacts LS-3a open.

When the heater assembly 24 returns to the solid line position shown inFIG. 1, the timer contacts Mlc close to energize the raise solenoid 98aof the cylinder 98 to move the frame 32 from the position shown in FIG.1 upwardly to a molding position adjacent the male mold 29. As the frameis moved upwardly the center portion of the sheet P will first engagethe bottom of the male mold 29 and then the portion of the sheetradially outward of the center portion will be rolled or draped upwardlyagainst the bottom of the male mold, thereby eliminating any voids andthe formation of air pockets between the sheet P and the plastic malemold member 29. When the mold box 32 reaches its uppermost position sothat the sheet P is in sealing engagement with the male mold member 29,the limit switch contacts LS5a are closed in line L-l6. In the interim,the clutch coil CL2 has been energized to close the contacts CL2a inline L-10 and initiate the forming timer M2. After the contacts LS-Saclose, the timer contacts M2a in line Ll6 close to energize the vacuumforming solenoid 170 which will connect the ducts 30 with a source ofvacuum so as to form the part against the male mold 29. As the carriage32 is moved upwardly to the forming position, the cam rollers 78 engagecams 79 on the frame F to move the side doors 92 and bottom doors 60from the solid line positions shown in FIGS. 6 and 7 to the chain linepositions shown in FIGS. 6 and 7. After the timer contacts M2a have beenclosed, the contacts M21; closed to energize the armatures 58a of themotors 58 so as to direct cooling air through the open doors 60, againstthe part formed, and thence through the open doors 92. After the parthas been sufficiently cooled, the contacts M2a close to energize thesolenoid 172 which disconnects the ducts 30 from the vacuum source andconnects them with a source of pressurized air for directing air throughthe ports 30 so as to eject the part from the male mold 29. As thecontacts M2c are closed, the contacts Mlc in line L-l5 are opened todeenergize the solenoid 98a and permit the carriage 32 to be loweredunder its own weight. As the carriage 32 lowers, the rods 80 are alsolowered until such time as the stops 81 engage the channel frame members110 to interrupt movement of the rods 80. The male box frame 32continues to move downwardly, however, and the cross members 82 engagethe bottom of the part formed to move the part upwardly relative to theframe 32. At this time, the limit switch LS-l is actuated to open thecontacts LS-Ia in Line L-3 to deenergize the solenoid 45 in line L5,thus moving the cylinders 40 and clamping bars 44 to their inoperativepositions, permitting the part and sheet P to be removed from the clampbox 32. The complete cycle, when forming a part in a sheet 0.080 inchesthick, takes approximately 40 seconds.

It should be understood, of course, that when the clamp box 32 is movingupwardly, the heating oven 24 was moving to the forming station 20 toheat the sheet of plastic supported on the male mold box frame 32'. Thesequence of operation for forming a part at station 20 is the same asthat described above for forming a part at station 18. When the box 32reaches its lowermost position, the switches 81-84 are opened and thesystem is in position so that the operation can again be repeated. 7

It is to be understood that the drawings and descriptive matter are inall cases to be interpreted as merely illustrative of the principles ofthe invention, rather than limiting the same in any way, since it iscontemplated that various changes may be made in various elements toachieve like results without departing from the spirit of the inventionor the scope of the appended claims.

I claim: 1. Differential pressure thermoforming apparatus comprising: 7

support means; first and second laterally spaced sheet supporting boxframe means on said support means each including means for supportingthe edge portions of a sheet of thermoplastic material;

first and second differential pressure mold means on said support meansin juxtaposition with said first and second sheet supporting box framemeans for forming shapes in sheets of thermoplastic material;

means on said support means for alternately relatively moving said firstand second sheet supporting box frame means and said first and secondmold means, respectively, between removed positions and operativemolding positions so that sheets of thermoplastic material on said firstand second sheet supporting box frame means are brought into engagementwith said first and second mold means to mold shapes in said sheets;

means for alternately introducing fluid under pressure to said first andsecond sheet supporting box frame means to support the mid-portions ofthe heated sheets on said first and second sheet supporting box framemeans as said first and second sheet supporting box frame means and saidfirst and second mold means, respectively, are relatively moved to saidoperative molding positions; and heating means movable on said supportmeans alternately to a first position between said first sheetsupporting box frame means and said first mold means in said removedpositions and a second position between said second sheet supporting boxframe means and said second mold means in said removed positions to heatsheets of material supported on said first and second box frame means,respectively, and while the mid-portions of said sheets are supported bysaid pressurized fluid.

2. The differential pressure thermoforming apparatus as set forth inclaim 1 wherein said pressurized fluid. introducing means is operativeto billow the midportions of clamped sheets outwardly away from saidsheet supporting means in a direction toward said mold means.

3. The differential pressure thermoforming apparatus as set forth inclaim 2 including sensing means for sensing the position of the billowedmid-portion of said sheets; and means responsive to said sensing meansfor interrupting said pressurized fluid introducing means when saidsheets are billowed outwardly a predetermined distance in a directionaway from said sheet supporting box frame means.

4. The differential pressure thermoforming apparatus of claim 1 whereineach of said sheet supporting box frame means defines a space spanned bya sheet supported thereon; said pressurized fluid being admitted to saidspaces; and means for introducing cooling air to said spaces to coolshapes molded in said sheets.

5. The differential pressure thermoforming apparatus of claim 4 whereineach of said sheet supporting box frame means includes exhaust portmeans; means for opening and closing said exhaust port means; andmeansfor moving said exhaust port opening and closing means to anexhaust port opening position so that the cooling air introduced to saidspaces may be exhausted through said exhaust port means.

6. The differential pressure thermoforming apparatus of claim 1 whereineach of said first and second sheet supporting box frame means defines aspace spanned by a sheet supported thereon; and including means disposedin said spaces and being movable relative to said sheet supporting boxframe means for supporting the mid-portions of said sheets when said boxframe means and said mold means are in said removed positions and forejecting the parts formed in the sheets from the mold box frame meanswhen said mold box frame means and said mold means are moving from saidoperative molding positions to said inoperative removed positions.

7. The differential pressure thermoforming apparatus of claim 6 whereinsaid means for supporting and ejecting comprises a plurality of loadassist bars disposed in each space and being movable relative to saidbox frame means; said pressurized fluid introducing means being operableto lift the mid-portions of the sheets of material upwardly away fromsaid load assist bars.

8. The differential pressure thermoforming apparatus as set forth inclaim 1 wherein means is provided on said support for reorienting saidheating meansrelative to said first and second sheet supporting meanswhen said heating means is in said first and second positions,respectively, so that the portions of the sheets of thermoplastic onsaid first and second frame means remote from said second and first moldmeans'respectively, are nearer to the heating means than the portions ofthe sheets on said first and second frame means closest to said secondand first mold means respectively.

9. Differential pressure thermoforming apparatus comprising:

aframe;

' sheet supporting means for supporting sheets of thermoplasticmaterial;

mold means on said frame disposed in juxtaposition with said sheetsupporting means for forming parts in the sheets supported thereon;

heating means for heating said sheets supported by said sheet supportingmeans;

means mounting said sheet supporting means=and said heating means forrelative movement between laterally adjacent positions, in which saidheating means is positioned laterally adjacent on lateral side of saidsheet supporting means, and juxtaposed positions;

means for relatively moving said sheet supporting means and said heatingmeans between said laterally adjacent positions and saidjuxtaposedpositions;

means for automatically tilting said heating means in said juxtaposedposition so that said heating means is nearer to the portion of thesheet adjacent the laterally opposite side of said sheet supportingmeans than to the portion of the sheet adjacent said one side of saidsheet supporting means so that the sheet of thermoplastic material isheated to a uniform temperature; and

means for relatively moving said sheet supporting means and said moldmember when said heating means is in said adjacent position to move aheated sheet into engagement with said mold member to form a part.

10. The differential pressure thermoforming apparatus of claim 9 whereinsaid heating means is pivotally mounted on said frame and said tiltingmeans comprises cooperating cam means and cam follower means positionedon said heating means and said frame which cooperate when said heatingmeans and said sheet supporting meansare in juxtaposition to pivot theheating means relative to said frame.

11. The apparatus set forth in claim 9 wherein said mold means comprisesfirst and second laterally spaced differential pressure mold members andsaid sheet supporting box frame means comprises laterally spaced firstand second sheet supporting box frames in juxtaposition with said firstand second mold members, said means for relatively moving said sheetsupporting means and said heating means comprises means for moving saidheating means alternately to a first position between said first moldmember and said first sheet supporting box frame and a second positionbetween said second sheet supporting box frame and said second moldmember to heat sheets supported on the first and second box framesrespectively.

12. The differential pressure thermoforming apparatus of claim 10wherein said heating means comprises resilient centering means onopposite sides of the pivotal mounting for yieldably urging said heatingmeans to a positionparallel with a sheet on said sheet supporting means,said cooperating cam means and cam follower means being operable toswing said heating means about said pivotal mounting when said heatingmeans and said sheet supporting means are in said juxtaposed positions.

13. The differential pressure-forming apparatus as set forth in claim 11wherein said heating means includes a carriage and a plurality ofheating elements on said carriage; said means for relatively moving saidsheet supporting means and said heating means includes means for movingsaid carriage from a position laterally adjacent said sheet supportingmeans to a superposed position with said sheet supporting means.

14. The differential pressure forming apparatus as set forth in claim 13wherein said means for relatively moving said mold means and said sheetsupporting means comprises rack means mounted on said support, andsprocket means disposed on opposite sides of said box frame drivinglyconnected to each other and in driving engagement with said rack'means.

l5. Differential pressure thermoforming apparatus comprising:

support means; sheet supporting box frame means on said support meansfor supporting the edge portions of a sheet of thermoplastic materialand having sides defining a space spanned by a sheet supported thereon;

differential pressure mold means mounted on said support means forforming a shape in said sheet of thermoplastic material;

means on said support means for moving said sheet supporting means froma removed position to an operative shape forming position adjacent saidmold means whereby said sheet is moved into engagement with said moldmeans to form a shape and for returning said sheet supporting means tosaid removed position to withdraw the shape from said mold means; and

means disposed in said space between the sides of said box frame meansfor supporting the mid-portion of said sheet when said box frame meansis in said removed position and for ejecting a shape formed in the sheetfrom said box frame means when said box frame means ismoving from saidoperative shape forming position to said removed position.

16. The differential pressure thermoforming apparatus of claim 15wherein said means for supporting and ejecting comprises a plurality ofload assist bars disposed in said space and being movable relative tosaid box frame means.

17. Differential pressure thermoforming apparatus comprising:

support means; I

a sheet supporting box frame on said support means for supporting asheet of thermoplastic material and defining a space spanned by saidsheet of material, said frame including an exhaust opening communicatingwith said space;

closure means for said exhaust opening movable between an open positionuncovering said exhaust opening and a closed position so as to coversaid exhaust opening;

differential pressure mold means on said support means for forming ashape in a sheet of thermoplastic material supported on said sheetsupporting box frame;

means for relatively moving said. sheet supporting box frame and saidmold means between removed inoperative positions and adjacent operativemolding positions to form a shape in said sheet of material; and

means for cooling a shape formed in said sheet comprising means forintroducing cooling air into said space and means for moving saidclosure means from said closed position to said open position to permitthe cooling air admitted to said space to be exhausted through saidexhaust opening.

18. The differential pressure thermoforming apparatus as set forth inclaim 17 including means yieldably urging said closure means to saidclosed position.

19. The differential pressure thermoforming apparatus of claim 18wherein said closure means comprises a door pivotally mounted on saidsheet supporting box frame; said means for moving said closure meanscomprises cooperating cam means and cam follower means connected withsaid door and said support means for pivoting said door to said openposition when said sheet supporting box frame and said mold means are insaid operative molding position against the force of said yieldablemeans.

20. The differential pressure thermoforming apparatus as set forth inclaim 17 wherein said sheet supporting box frame includes a cooling airinlet opening; and means for said inlet opening selectively movablebetween an open position removed from said inlet opening and a closedpositions so as to cover said inlet opening; said means for introducingcooling air includes means for moving said second closure means betweensaid closed and open positions; and fan means for drawing cooling airinto said space when said second closure means is in said open position.

21. The differential pressure thermoforming apparatus as set forth inclaim 20 wherein said means for selectively moving said second closuremeans to said open position to permit cooling air to enter through saidinlet opening comprises a door pivotally mounted on said sheetsupporting box frame, and cooperating cam means and cam follower meansmounted on said box frame and said support means cooperating to movesaid door to said open position when said box frame is in said operativeposition.

22. The differential pressure thermoforming apparatus as set forth inclaim 21 wherein said means for moving said first mentioned closuremeans is movable simultaneously with said means for moving said secondclosure means and comprises a shaft connected with said means for movingsaid first mentioned closure means and said second closure means tosimultane ously move said first mentioned closure means and said secondclosure means to their open positions.

1. Differential pressure thermoforming apparatus comprising: supportmeans; first and second laterally spaced sheet supporting box framemeans on said support means each including means for supporting the edgeportions of a sheet of thermoplastic material; first and seconddifferential pressure mold means on said support means in juxtapositionwith said first and second sheet supporting box frame means for formingshapes in sheets of thermoplastic material; means on said support meansfor alternately relatively moving said first and second sheet supportingbox frame means and said first and second mold means, respectively,between removed positions and operative molding positions so that sheetsof thermoplastic material on said first and second sheet supporting boxframe means are brought into engagement with said first and second moldmeans to mold shapes in said sheets; means for alternately introducingfluid under pressure to said first and second sheet supporting box framemeans to support the mid-portions of the heated sheets on said first andsecond sheet supporting box frame means as said first and second sheetsupporting box frame means and said first and second mold means,respectively, are relatively moved to said operative molding pOsitions;and heating means movable on said support means alternately to a firstposition between said first sheet supporting box frame means and saidfirst mold means in said removed positions and a second position betweensaid second sheet supporting box frame means and said second mold meansin said removed positions to heat sheets of material supported on saidfirst and second box frame means, respectively, and while themid-portions of said sheets are supported by said pressurized fluid. 1.Differential pressure thermoforming apparatus comprising: support means;first and second laterally spaced sheet supporting box frame means onsaid support means each including means for supporting the edge portionsof a sheet of thermoplastic material; first and second differentialpressure mold means on said support means in juxtaposition with saidfirst and second sheet supporting box frame means for forming shapes insheets of thermoplastic material; means on said support means foralternately relatively moving said first and second sheet supporting boxframe means and said first and second mold means, respectively, betweenremoved positions and operative molding positions so that sheets ofthermoplastic material on said first and second sheet supporting boxframe means are brought into engagement with said first and second moldmeans to mold shapes in said sheets; means for alternately introducingfluid under pressure to said first and second sheet supporting box framemeans to support the mid-portions of the heated sheets on said first andsecond sheet supporting box frame means as said first and second sheetsupporting box frame means and said first and second mold means,respectively, are relatively moved to said operative molding pOsitions;and heating means movable on said support means alternately to a firstposition between said first sheet supporting box frame means and saidfirst mold means in said removed positions and a second position betweensaid second sheet supporting box frame means and said second mold meansin said removed positions to heat sheets of material supported on saidfirst and second box frame means, respectively, and while themid-portions of said sheets are supported by said pressurized fluid. 2.The differential pressure thermoforming apparatus as set forth in claim1 wherein said pressurized fluid introducing means is operative tobillow the mid-portions of clamped sheets outwardly away from said sheetsupporting means in a direction toward said mold means.
 3. Thedifferential pressure thermoforming apparatus as set forth in claim 2including sensing means for sensing the position of the billowedmid-portion of said sheets; and means responsive to said sensing meansfor interrupting said pressurized fluid introducing means when saidsheets are billowed outwardly a predetermined distance in a directionaway from said sheet supporting box frame means.
 4. The differentialpressure thermoforming apparatus of claim 1 wherein each of said sheetsupporting box frame means defines a space spanned by a sheet supportedthereon; said pressurized fluid being admitted to said spaces; and meansfor introducing cooling air to said spaces to cool shapes molded in saidsheets.
 5. The differential pressure thermoforming apparatus of claim 4wherein each of said sheet supporting box frame means includes exhaustport means; means for opening and closing said exhaust port means; andmeans for moving said exhaust port opening and closing means to anexhaust port opening position so that the cooling air introduced to saidspaces may be exhausted through said exhaust port means.
 6. Thedifferential pressure thermoforming apparatus of claim 1 wherein each ofsaid first and second sheet supporting box frame means defines a spacespanned by a sheet supported thereon; and including means disposed insaid spaces and being movable relative to said sheet supporting boxframe means for supporting the mid-portions of said sheets when said boxframe means and said mold means are in said removed positions and forejecting the parts formed in the sheets from the mold box frame meanswhen said mold box frame means and said mold means are moving from saidoperative molding positions to said inoperative removed positions. 7.The differential pressure thermoforming apparatus of claim 6 whereinsaid means for supporting and ejecting comprises a plurality of loadassist bars disposed in each space and being movable relative to saidbox frame means; said pressurized fluid introducing means being operableto lift the mid-portions of the sheets of material upwardly away fromsaid load assist bars.
 8. The differential pressure thermoformingapparatus as set forth in claim 1 wherein means is provided on saidsupport for reorienting said heating means relative to said first andsecond sheet supporting means when said heating means is in said firstand second positions, respectively, so that the portions of the sheetsof thermoplastic on said first and second frame means remote from saidsecond and first mold means respectively, are nearer to the heatingmeans than the portions of the sheets on said first and second framemeans closest to said second and first mold means respectively. 9.Differential pressure thermoforming apparatus comprising: a frame; sheetsupporting means for supporting sheets of thermoplastic material; moldmeans on said frame disposed in juxtaposition with said sheet supportingmeans for forming parts in the sheets supported thereon; heating meansfor heating said sheets supported by said sheet supporting means; meansmounting said sheet supporting means and said heating means for relativemovement between laterally adjacent positions, in which said heatingmeAns is positioned laterally adjacent on lateral side of said sheetsupporting means, and juxtaposed positions; means for relatively movingsaid sheet supporting means and said heating means between saidlaterally adjacent positions and said juxtaposed positions; means forautomatically tilting said heating means in said juxtaposed position sothat said heating means is nearer to the portion of the sheet adjacentthe laterally opposite side of said sheet supporting means than to theportion of the sheet adjacent said one side of said sheet supportingmeans so that the sheet of thermoplastic material is heated to a uniformtemperature; and means for relatively moving said sheet supporting meansand said mold member when said heating means is in said adjacentposition to move a heated sheet into engagement with said mold member toform a part.
 10. The differential pressure thermoforming apparatus ofclaim 9 wherein said heating means is pivotally mounted on said frameand said tilting means comprises cooperating cam means and cam followermeans positioned on said heating means and said frame which cooperatewhen said heating means and said sheet supporting means are injuxtaposition to pivot the heating means relative to said frame.
 11. Theapparatus set forth in claim 9 wherein said mold means comprises firstand second laterally spaced differential pressure mold members and saidsheet supporting box frame means comprises laterally spaced first andsecond sheet supporting box frames in juxtaposition with said first andsecond mold members, said means for relatively moving said sheetsupporting means and said heating means comprises means for moving saidheating means alternately to a first position between said first moldmember and said first sheet supporting box frame and a second positionbetween said second sheet supporting box frame and said second moldmember to heat sheets supported on the first and second box framesrespectively.
 12. The differential pressure thermoforming apparatus ofclaim 10 wherein said heating means comprises resilient centering meanson opposite sides of the pivotal mounting for yieldably urging saidheating means to a position parallel with a sheet on said sheetsupporting means, said cooperating cam means and cam follower meansbeing operable to swing said heating means about said pivotal mountingwhen said heating means and said sheet supporting means are in saidjuxtaposed positions.
 13. The differential pressure forming apparatus asset forth in claim 11 wherein said heating means includes a carriage anda plurality of heating elements on said carriage; said means forrelatively moving said sheet supporting means and said heating meansincludes means for moving said carriage from a position laterallyadjacent said sheet supporting means to a superposed position with saidsheet supporting means.
 14. The differential pressure forming apparatusas set forth in claim 13 wherein said means for relatively moving saidmold means and said sheet supporting means comprises rack means mountedon said support, and sprocket means disposed on opposite sides of saidbox frame drivingly connected to each other and in driving engagementwith said rack means.
 15. Differential pressure thermoforming apparatuscomprising: support means; sheet supporting box frame means on saidsupport means for supporting the edge portions of a sheet ofthermoplastic material and having sides defining a space spanned by asheet supported thereon; differential pressure mold means mounted onsaid support means for forming a shape in said sheet of thermoplasticmaterial; means on said support means for moving said sheet supportingmeans from a removed position to an operative shape forming positionadjacent said mold means whereby said sheet is moved into engagementwith said mold means to form a shape and for returning said sheetsupporting means to said removed position to withdraw the shape fromsaid mold means; and meaNs disposed in said space between the sides ofsaid box frame means for supporting the mid-portion of said sheet whensaid box frame means is in said removed position and for ejecting ashape formed in the sheet from said box frame means when said box framemeans is moving from said operative shape forming position to saidremoved position.
 16. The differential pressure thermoforming apparatusof claim 15 wherein said means for supporting and ejecting comprises aplurality of load assist bars disposed in said space and being movablerelative to said box frame means.
 17. Differential pressurethermoforming apparatus comprising: support means; a sheet supportingbox frame on said support means for supporting a sheet of thermoplasticmaterial and defining a space spanned by said sheet of material, saidframe including an exhaust opening communicating with said space;closure means for said exhaust opening movable between an open positionuncovering said exhaust opening and a closed position so as to coversaid exhaust opening; differential pressure mold means on said supportmeans for forming a shape in a sheet of thermoplastic material supportedon said sheet supporting box frame; means for relatively moving saidsheet supporting box frame and said mold means between removedinoperative positions and adjacent operative molding positions to form ashape in said sheet of material; and means for cooling a shape formed insaid sheet comprising means for introducing cooling air into said spaceand means for moving said closure means from said closed position tosaid open position to permit the cooling air admitted to said space tobe exhausted through said exhaust opening.
 18. The differential pressurethermoforming apparatus as set forth in claim 17 including meansyieldably urging said closure means to said closed position.
 19. Thedifferential pressure thermoforming apparatus of claim 18 wherein saidclosure means comprises a door pivotally mounted on said sheetsupporting box frame; said means for moving said closure means comprisescooperating cam means and cam follower means connected with said doorand said support means for pivoting said door to said open position whensaid sheet supporting box frame and said mold means are in saidoperative molding position against the force of said yieldable means.20. The differential pressure thermoforming apparatus as set forth inclaim 17 wherein said sheet supporting box frame includes a cooling airinlet opening; and means for said inlet opening selectively movablebetween an open position removed from said inlet opening and a closedpositions so as to cover said inlet opening; said means for introducingcooling air includes means for moving said second closure means betweensaid closed and open positions; and fan means for drawing cooling airinto said space when said second closure means is in said open position.21. The differential pressure thermoforming apparatus as set forth inclaim 20 wherein said means for selectively moving said second closuremeans to said open position to permit cooling air to enter through saidinlet opening comprises a door pivotally mounted on said sheetsupporting box frame, and cooperating cam means and cam follower meansmounted on said box frame and said support means cooperating to movesaid door to said open position when said box frame is in said operativeposition.